U.S. Pat. No. 6,066,072 discloses lockup control of a lockup clutch associated with a torque converter for a vehicle. The torque converter connects an engine and an automatic transmission of the vehicle and comprises a pump impeller connected to the engine and a turbine runner connected to the transmission. An output torque of the engine is transmitted to the transmission via a fluid between the pump impeller and the turbine runner. The lockup clutch engages the pump impeller with the turbine runner such that the torque is transmitted directly.
The ratio of a torque transmitted via fluid and a torque transmitted via the lockup clutch depends on an engaging force applied to engage the lockup clutch. When the engaging force is the minimum, the entire torque is transmitted via fluid, and when the engaging force is the maximum, the entire torque is transmitted via the lockup clutch.
According to the prior art, when the lockup clutch is engaged after the vehicle starts, the engaging force is first controlled by an open loop control. As the engaging force increases, a relative rotation speed of the pump impeller and turbine runner of the torque converter decreases.
According to the prior art, when the relative rotation speed decreases to a predetermined relative rotation speed, control of the engaging force is changed over from the open loop control to a feedback/feedforward control.
The feedback/feedforward control of the engaging force is constituted by a feedback control and a feedforward control. In the feedback/feedforward control, a target relative rotation speed is determined according to engine running conditions. The feedback control of the engaging force is performed so as to decrease a difference between the target relative rotation speed and the real relative rotation speed. The feedforward control of the engaging force is performed in combination with the feedback control so as to increase the engaging force to a feedforward control value determined according to the target relative rotation speed. The feedforward control is implemented to enhance a control response of the engaging force.